THE EFFECT OF STRUCTURE AND MORPHOLOGY ON THE LIGHT TRANSMISSIVITY OF MINERALIZED SPINES OF LANTERNSHARKS

Abstract

In the darkness of the deep ocean a surprising diversity of organisms generate their own light to find food, communicate or avoid predation. The velvet belly lanternshark, Etmopterus spinax, from one of the few families of luminous sharks, emits a blue glow from thousands of tiny photophores embedded in its skin. Etmopterus, like most luminous and non-luminous dogfish sharks, possesses defensive, mineralized spines on the leading edges of their two dorsal fins; in this species, however, the spine tips appear to glow from the photophores localized behind them. In order to characterize the interaction of Etmopterus spines with their light production we correlated light transmission properties with aspects of gross- and ultrastructural morphology from spines of closely-related luminous and non-luminous dogfish. Luminometry experiments —in which a laser of luminescence-wavelength (470nm) was shone through spines— illustrate that Etmopterusspines transmit 60% of incident light, in contrast to only 10% through spines from a non-luminescent species (Squalus). In both species, the single light source was split into two components when passing through the spine, with those of Etmopterus oriented more anteriorly than those of those of Squalus. Quantitative modeling based on luminometry results, in vivo measures of light from spine-associated photophores, and in situ measures of ambient light indicate that spine-associated light would be visible from several meters away in the shark’s natural environment. Species’ difference in transmissivity is reflected in both the cross-sectional shape and mineral content of spines, quantified through microCT scans and backscatter electron imaging (SEM-BEI) of spines from three Squalus, four Etmopterus, and one outgroup. Whereas the tips of all spines were comparatively highly mineralized, spines of Squalus exhibited an absolute higher level and more uniform distribution of mineral content. Also, in comparison with those of non-luminescent species, which tend to be an inverted heart-shape in cross-section, cross-sections of Etmopterus spines are half-ovals, with flattened incident faces and convex refractive faces. A raised ridge on the spine forms an additional and smaller ovular cross-section anterior to the spine’s large primary chamber; we discuss and model the possibility of this ancillary chamber acting as a secondary lens and whether the pressures of mechanical integrity for defensive purposes are competing factors with those for light transmission. These results, in demonstrating correlations between differences in gross-geometry, mineral content, and light transmissivity amount and orientation, support our hypothesis that spine-associated luminescence acts as an aposematic signal, warning predators away. The presence of distinct structural features associated only with luminescence, suggests that these features co-evolved, rather than spine morphology being pre-adapted for light transmission.